One of the primary components in paint is the "film former" that provides a film for the protective function for a substrate coated with the paint. Film forming components of liquid paints include resins which have required organic solvents to provide the resins with suitable viscosities such that the paint can be applied by existing commercial application equipment. Use of organic solvents, however, raises at least two problems. In the past and potentially in the future, petrochemical shortages mitigate against the use of organic solvent in great volumes. Second, environmental concern mitigates against the use of organic solvents and requires such use be minimized.
Environmental concern has become increasingly important. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to VOCs which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and industrial coating industry which uses organic solvents which are emitted into the atmosphere.
To reduce organic solvent content and VOCs, researchers have developed high solids coating compositions and powdered coating compositions. High solids compositions generally are liquid and are designed to minimize solvents. Powdered coating compositions are solid powders and generally eliminate solvents. While each have advantages, each coating composition has disadvantages.
Coating compositions which include high solids polymeric vehicles based upon polyesters have become popular. Typically these compositions require the use of some organic solvents. In high solid polyesters as opposed to "conventional" compositions which use organic solvents, high molecular weight generally needs to be achieved during crosslinking rather than being attained from the basic polyester polymer. Hence, high solids polyesters normally supply a greater number of reactive sites (predominantly hydroxyl groups) available for crosslinking. The resultant polymers typically exhibit 70-80% solids-weight when reacted stoichiometrically with isocyanate crosslinkers, but frequently yield empirical solids up to 12% lower, when crosslinked with melamine resins. Despite their reduced use of organic solvents, high solids polyester coating compositions can be produced on the same equipment and be employed in many of the same applications as lower solids "conventional" polyester coating compositions. Further, as a result of their many strengths such as ease of manufacturing and use, low volatile emissions, reduced energy requirements, greater application efficiency, lower handling and storage costs, and excellent physical properties, high solids polyester coating compositions have enjoyed spectacular growth in manufacture and use. They still require organic solvents, however, and are a source of VOCs.
Powder coatings and UV-curable coatings are desirable ultrahigh or 100% solids coatings. However, there are limitations as to the technique and the equipment which are used to apply the powdered composition.
To reduce solvent content and VOCs in polymeric vehicles and formulated coating compositions for paints, researchers have been driven by three major objectives: controlling the reactivity of the film forming components in the paint; keeping the viscosity of the components in the paint low to minimize the organic solvents in the paint and to keep the VOCs in the paint at the lowest possible level; and keeping the components in the paint at a low volatility to minimize VOCs.
One way to reduce VOCs is to further reduce organic solvent content and increase the solids level of the coating composition to an ultra high solids level. High viscosity is a major problem which needs to be solved in ultrahigh solids coatings. In high solids polyester coatings, the viscosity of concentrated polyester solutions depends on several variables. Molecular weight and molecular weight distribution are two important factors. According to polymer physics theory, the viscosity of polymers in the liquid state depends mainly on the average molecular weight and the temperature, so it is desirable to reduce average molecular weight for solventless polyester coating. The major factor controlling number average molecular weight (M.sub.n) of a polyester is the mole ratio of dibasic acid/diol or polyol. A dibasic acid to diol or polyol ratio of the order of 2:3 is typical. However, loss of polyol during the production of the polyester can result in a significantly higher number average molecular weight than predicted from the starting ratio. It is necessary to add some extra glycol to compensate for loss. Further, in ultra high solids coatings, the low molecular weight fraction of resin in the polymeric vehicle may be volatile enough to evaporate when a thin film is baked. Such loss has to be counted as part of the VOC emissions. Further, ultra-high solids polymeric vehicles also may include organic solvents and VOCs.
The number of functional groups per molecule also affects the viscosity because of hydrogen bonding. Most oligomers or polymers require high functionality to achieve a highly crosslinked film and reasonable glass transition temperatures (Tgs) to have adequate film properties for most applications. The high functionality tends to increase the viscosity significantly.
An object of the invention is to provide a polymeric vehicle which will reduce or eliminate VOCs.
Another object of this invention is to provide polymeric vehicles which are not only low in VOCs, but which provide coating binders with good film properties such as hardness and impact resistance.
Yet another object of this invention is to control the viscosity to low levels at a specific shear rate of a liquid polymeric vehicle or liquid formulated coating composition with the use of water and without using organic solvents for such control.
Further, objects and advantages of the invention will be found by reference to the following description.
SUMMARY OF THE INVENTION
The invention provides a liquid polymeric vehicle which is a solution and which polymeric vehicle is effective for providing an aqueous high solids polymeric vehicle or formulated coating composition which has at least 3 weight percent water, based upon the weight of the formulated coating composition. The polymeric vehicle comprises an oligoester diol having a polydispersity index of not greater than about 2.6, a carbamate hardener, and in an important aspect, an additional amino type crosslinker resin.
Broadly, the oligoester diol generally has a molecular weight and structure effective for providing a liquid formulated coating composition or liquid polymeric vehicle with a viscosity of less than about 1.5 Pa.s at of temperature of from about 20.degree. C. to about 60.degree. C. at a shear rate of about three sec..sup.-1. These viscosities are attained when the oligoester diol is combined with at least about three weight percent water, based upon the weight of the formulated coating composition, the carbamate hardener, and not more than three weight percent, based upon the weight of the formulated coating composition, organic solvent. In an important aspect, the formulated coating composition and polymeric vehicle do not have more than about three weight percent organic solvent and will imbibe at least eight weight percent water. In a very important aspect, the polymeric vehicle and formulated coating composition are solventless, that is without organic solvent except in trace amounts (less than 0.25%).
In a very important aspect of the invention, the oligoester diol is selected from the group consisting of a substantially linear oligoester diol, a linear oligoester diol and mixtures thereof. When the carbamate hardener is selected from the group consisting of a blocked polyisocyanate, a polyisocyanate, a blocked polyfunctional biuret, a polyfunctional biuret, a carbamate polyol, and mixtures thereof, it is reactive with the oligoester diol, or reactive with an additional crosslinker which is reactive with the oligoester diol. Alternatively, when the carbamate hardener is a polyisocyanate or carbamate polyol, it is reactive with the blocked polyisocyanate, blocked polyfunctional biuret as well as one or more additional amino type crosslinkers which also are reactive with the oligoester diol. In either case, the carbamate hardener is miscible with, but is not reactive with water.
The carbamate hardener and oligoester diol are in relative amounts effective for permitting the polymeric vehicle or the formulated coating composition to imbibe at least about 3 weight percent water, based upon the weight of the formulated coating composition and for providing a water thinned formulated coating composition with a viscosity of less than about 1.5 Pa.s, preferably about 1.2 Pa.s, at about 25.degree. C. at a shear rate of about 3 sec..sup.-1. In an important aspect, the carbamate hardener and oligoester diol are in relative amounts effective for permitting the polymeric vehicle or the formulated coating composition to imbibe at least eight percent and preferably at least about 10 weight percent water to provide a polymeric vehicle effective for providing a water thinned formulated coating composition with the latter viscosities at 25.degree. C. The polymeric vehicle of the invention also is effective for providing a coating binder having a hardness of at least about H when applied to and thermoset on a substrate at a thickness of one mil dry.
The combination of the substantially linear and/or linear oligoester diol with the carbamate hardener provides a low viscosity coating composition with a low (not more than about 200 g/L) to about zero VOC content. The use of the carbamate hardener in combination with the oligoester diol permits the addition of water to the carbamate hardener/oligoester diol blend to further reduce the viscosity of the polymeric vehicle without increasing the VOC content thereof.
The viscosity of the oligoester diol is generally a function of its linearity, the more linear the diol, the lower the viscosity of the diol. Further, the viscosity of the oligoester diol and the VOCs which potentially emanate from the oligoester diol in the polymeric vehicle are a function of the polydispersity index (PDI) of the oligoester diol. Keeping the polydispersity index low, such as below about 2.6, preferably below about 2.2, and most preferably below about 1.4, helps control the viscosity of the oligoester diol, and hence, the viscosity of the polymeric vehicle and the formulated coating composition. High PDIs result in relatively high fractions of high molecular weight oligoester diols which increase viscosity beyond what would be ratably expected from the high molecular weight fraction of oligoester diol. Further, high PDIs also mean fractions of low molecular weight oligoester diol that would contribute to VOCs of the polymeric vehicle as a result of the low molecular fraction of the oligoester diol not being reacted and being made a part of the coating binder. These low molecular weight fractions evaporate rather than react in the thermosetting curing reaction for the coating binder, and as a result, may be considered a VOC.
The linearity of the oligoester diol may be decreased without effecting an increase of the viscosity of the formulated coating composition and without a deleterious effect on the properties of coating binder by using water as a thinning agent or solvent. The amount of water used is a function of the linearity (or nonlinearity) of the oligoester diol; and hence, the viscosity of the oligoester diol. In general, the more linear the oligoester diol, the less the viscosity of that diol at a given temperature, M.sub.n and PDI. At least about 3 weight percent of water, and in an important aspect at least eight weight percent water, may be added to reduce the viscosity of the formulated coating composition or polymeric vehicle with a relatively linear oligoester diol and with a carbamate hardener which is a polyisocyanate, blocked polyisocyanate or blocked polyfunctional biuret. To increase the amount of water in the system, to reduce the amount of polyisocyanate crosslinkers in the system, to increase the amount of nonlinear oligoester diol in the system (which also will increase the viscosity of the polymeric vehicle and formulated coating composition without the use of additional solvent) and to permit the use of other crosslinkers in the polymeric vehicle and formulated coating composition without a deleterious effect on the properties of the coating binder, carbamate hardener, other than blocked or unblocked polyisocyanates and biurets, may be added to the polymeric vehicle and formulated coating composition. These latter non-isocyanate carbamate hardeners are urethane polyols having a molecular weight of not more than about 1200 and a neat viscosity of not more than about 25 Pa.s at 25.degree. C.
In general the viscosity of the polymeric vehicle which includes a blend of the oligoester diol and carbamate hardener will be in the range of from about 0.6 to about 10 Pa.s at from about 20.degree. C. to about 60.degree. C. at a shear rate of about 3 sec..sup.-1 in the absence of organic solvent and water. The amounts of oligoester diol and carbamate hardener and optionally one or more additional amino type resin crosslinkers in the polymeric vehicle will be in relative amounts to provide a viscosity of the blend in the range as aforesaid, and preferably a viscosity in the range of about 0.6 to about 8.0 Pa.s in the absence of organic solvent and water. In the aspect of the invention which uses only oligoester diol and blocked or unblocked polyisocyanate and/or biuret as the carbamate hardener, the polymeric vehicle comprises from about 25 to about 80 weight percent blocked or unblocked polyisocyanate and/or blocked or unblocked polyfunctional biuret carbamate hardener, based upon the combined weight of the oligoester diol and carbamate hardener, and from about 20 to about 75 weight percent oligoester diol. If the carbamate hardener includes a polyurethane polyol such as a polyurethane diol with the use of a crosslinker and optionally at least one blocked or unblocked polyisocyanate or blocked or unblocked polyfunctional biuret crosslinker, the polyurethane diol and polyisocyanate or polyfunctional biuret are present in an amount effective to provide the coating binder with from about 3 to about 20 weight percent carbamate groups [--OC(.dbd.O)N(--H)--], based upon the weight of the coating binder.
The oligoester diol has a number average molecular weight in the range of from about 275 to about 3000 and, in an important aspect, is linear or substantially linear. The oligoester diol has a main longitudinal chain which has chain segments having the structures --CH.sub.2 --, --O--, --C(.dbd.O)--. When the oligoester diol is substantially linear, to a limited extent it may have segments with structures --(R)CH-- and --(R).sub.2 C-- wherein R is methyl or ethyl and the chain is terminated with hydroxyl groups. Generally R will be methyl.
In another aspect, the invention provides a method for controlling the viscosities of the polymeric vehicle and formulated coating composition at a specific shear rate by controlling the relative amounts of linear and substantially linear nonmesogenic oligoester diol and carbamate hardener such that when the oligoester diol and carbamate hardening are combined, the blend of the oligoester diol and carbamate hardener will be in relative amounts effective to provide (1) a water thinned formulated coating composition having at least 3 weight percent water, and in an important aspect, at least about 8 weight percent water, and having a viscosity of from about 0.1 to about 1.5 Pa.s at a temperature of from about 20.degree. C. to about 60.degree. C. at a shear rate of about 3 sec..sup.-1, and (2) a coating binder with a hardness of at least about H after a thermosetting cure at a binder thickness of about one mil dry.